Offshore installations, such as offshore windmills, various process modules for subsea oil and gas exploration etc., are in many cases deployed by being transported on seagoing transport vessels out to the placement location, and thereafter lifted off the transport vessel by an on-board crane or crane located on another vessel and lowered into the sea, either to be located on the surface as a floating installation or lowered into the water to be installed on the sea floor.
The deployment, or more precise, the lifting operation of the load is sensitive to the weather conditions because sea wave induced movements of the deployment vessel may quickly cause unacceptable heave movements of the suspended load. This is particularly problematic for lifting of heavy cargoes and/or huge constructions form floating vessels.
There are several problems associated with heave movements of heavy suspended loads. The heave movements are typically difficult to predict and has an irregular cyclic nature causing irregularly varying accelerated motions of the suspended load.
The accelerated motions may cause unacceptably high tensions on the lifting equipment/crane and suspension points both when the load is suspended in air, and in particular when the load is immersed and in water. Then the drag force from the water mass may easily result in unacceptably high tension forces on the lifting equipment.
Another problem with heave movements is that they are causing difficulties to predict the vertical motion of the load. This is problematic both at the initial lifting phase and at the landing phase of the load, due to risk of the load ramming into the basement or deployment vessel causing structural damages to the load and/or deployment vessel or intended landing basement.
A further problematic phase in offshore lifts of heavy loads and/or large constructions is the passing through the so-called splash zone, which is when the load/construction is partially submerged into water. In this phase, water/sea waves may induce changes in the buoyance of the load/construction causing temporarily slack in the lifting cable and/or slings which often subsequently ends with a snap load when the cable and/or slings is suddenly tightened. Snap loads are problematic due to easily giving unacceptable high tension forces which in the worst case may cause the cable or sling to snap.
Based on lift engineering calculations, a weather window is predicted where safe operations can be done according to acceptance criteria. This may significantly reduce operability and create long periods of waiting before the acceptable wave conditions are established.
Thus, to avoid costly waiting periods where the transport vessel is laying inactive waiting for improved wave conditions allowing such deploying operations, it is a desire for amending these wave induced problems allowing performing the deployment in less favourable weather conditions.
It is common to alleviate these problems by employing a heave compensator. A heave compensator is a mechanism having a spring and/or dampening effect due to being able, when needed, to prolong or shorten the distance between the suspension point of the crane and the suspension point of the load, and thus substantially reduce the variations of the tension forces due to unintended movements during the lift. The heave compensator is typically arranged between the load and the crane, e.g. by being attached in one end to a clevis of the lifting cable of the crane and in the other end to the suspension point of the load.
From U.S. Pat. No. 3,842,603, it is known a crane load compensator for interconnection between a stationary crane and a ship-borne load having a double acting piston dividing a cylinder into primary and secondary chambers. A first accumulator having liquid in the lower portion thereof and air in the portion thereof is connected at the bottom through a first electrically actuated valve to the lower one of the chambers. A second accumulator having liquid in the lower portion and air in the upper portion is connected at the bottom to the other chamber. The first accumulator also has a second electrically actuated valve for supplying air under pressure to it and a third electrically actuated valve for releasing air under pressure therefrom. The second accumulator has a first pressure switch responsive to low air pressure in the second accumulator connected to the second valve and has a second pressure switch responsive to high pressure in the second accumulator connected to the third valve. The pressure of the air is used as a piston position indicator. In addition, the first valve is controlled by a manually operated switch. Also, there is a manually operated switch for placing in circuit the actuators for the second and third air valves. By manipulation of the valves manually and automatically, the load is cushioned and handled carefully despite variations in position between the ship and the crane.
From EP 2 982 638 A1 it is disclosed a heave compensator with adjustable dampening properties comprising a length extension device having an inner space divided by a slide-able piston into a vacuum chamber and a liquid filled chamber, a gas accumulator divided by a slide-able piston into a gas filled chamber and a liquid filled chamber, and eventually a gas tank having an expansion chamber, where the liquid and gas chamber are fluidly connected to each other with valve controlled conduits, and where the device comprises pressure and temperature sensors which register pressure and temperature in the gas- and liquid phases, and where the device further comprises a control unit comprising a signal receiving unit, a writeable computer memory, a data processing unit, and a signal transmitting unit, and where the data processing unit contains computer software which calculates suited amount of gas and gas pressure in the at least one gas accumulator and/or at least one gas tank based on the information of which lifting operation is going to be performed and which thereafter engages activation means such that the suited amount of gas and gas pressure are achieved and maintained during the different phases of the lifting operation.
EP 2 982 636 discloses a heave compensator for heavy lifts with adjustable dampening characteristics able to operate above and below the water line in environmental pressures ranging from the atmospheric up to several hundred atmospheres pressure, and further to a method for automatic regulation of the available stroke length of the heave compensator during the lifting operation, based on the realization that heave compensating devices utilising a slide-able piston as a volume expanding mechanism to reduce the tension forces upon relative movements between crane and load, may obtain a simple compact construction able to execute a range of different compensation functionalities by registering the pressure and temperature in the gas filled chambers of the device, and employing this information to regulate the amount of gas in the single gas filled chambers.
From NO 2014 0672 it is disclosed a self-regulating heave compensator comprising a cylinder having a slide-able piston with piston rod extending out of the cylinder and where the piston divides the inner space of the cylinder into an upper vacuum chamber and a lower liquid filled chamber. The heave compensator comprises further at least a first and a second accumulator having a slide-able piston which divides their inner space into a lower liquid filled chamber and an upper gas-filled chamber, and at least a first and a second gas tank, and where the first gas tank is filled with gas at a relatively low pressure and the second gas tank is filled with gas at a relatively high pressure. A first closable (with a valve) fluid passage is fluidly connecting the liquid in the lower chamber of the cylinder with the liquid of the lower chamber of the first accumulator. A second fluid passage (without valve) ensures free fluid communication between the gases of the first accumulator and the second accumulator. A third closable (with a valve) a fluid passage enables tapping off liquid (to the environment) from the second accumulator. A fourth closable (with a valve) fluid passage connects the gas phases of the first accumulator to the first gas tank, and a fifth closable (with a valve) fluid passage connects the gas phases of the first accumulator to the second gas tank.
WO 2014/122527 discloses a passive heave compensator comprising: a main hydraulic cylinder, including a moveable piston having a piston rod extendible through the main hydraulic cylinder and a piston head, a gas phase above the piston head, and at least one oil phase below the piston head separated by a boundary; an upper connection point associated with the main hydraulic cylinder and a lower connection point associated with the piston rod; and at least one accumulator, each accumulator having a moveable separator to divide the accumulator between a gas phase above the separator, and an oil phase below the separator, and each oil phase is in communication with an oil phase in the main hydraulic cylinder; characterized in that the main hydraulic cylinder further comprises a cylinder sleeve co-axial with the piston head to provide, in co-ordination with the piston head, the boundary between the gas phase and the at least one oil phase in the main hydraulic cylinder. In this way, the variation in the coordination between the shape, longitudinal position, or both of the piston head, which naturally must be smaller in cross-section than the cross-section of the main hydraulic cylinder, and the transverse extent of the cylinder sleeve, provides variation in the cross-sectional area of oil volume in the main hydraulic cylinder, and thus different damping effects along the length of the main hydraulic cylinder, which are available to the user.
From U.S. Pat. No. 7,934,561 it is known a depth compensated passive heave compensator with depth compensation comprising a first cylinder connected at its upper end to a vessel. A piston rod extends from a piston located within the first cylinder through the lower end thereof and is connected to subsea equipment. A second cylinder contains a compressed gas which maintains pressure beneath the piston of the first cylinder. The upper end of the first cylinder is connected to the upper end of a third cylinder having a piston mounted therein. A piston rod extending from the piston of third cylinder extends through the lower end thereof, thereby applying the pressure of the sea to the piston of the third cylinder.
From U.S. 2005/0074296 it is known a hydro-pneumatic tensioner including a barrel having an inner bore and a pressurized fluid contained within to form at least part of a primary accumulator having a preset volume of gas at a preselected pressure. A piston having a piston rod extending from an aperture in the barrel is slideably carried in the bore of the barrel and is in communication with the pressurized fluid and positioned to increase the fluid pressure when the piston strokes in the direction of the pressurized fluid. A secondary accumulator also has a preset volume of gas at a preselected pressure. A fluid separator maintains functional separation of the fluid volumes of the primary and secondary accumulators when the primary accumulator pressure is less than a preselected secondary accumulator pressure. The fluid separator allows functional combining of the fluid volumes of the primary and secondary accumulators when the primary accumulator pressure equals or is greater than the preselected secondary accumulator pressure